.tab {
    height: auto;
    width: 100%;
    padding: 20px 30px;
}

.form_auth {
    position: relative;
    margin-bottom: 20px;
}

.form_auth label {
    position: absolute;
    top: 14px;
    left: 7px;
    padding: 0 5px;
    /* 给top和font-size 添加动画效果 */
    transition: top .5s, font-size .3s;
    color: #808080;
    background-color: #f0f0f0;
}

.form_auth input {
    box-sizing: border-box;
    height: 45px;
    padding: 0 12px;
    color: #080808;
    background-color: #f0f0f0;
    width: 240px;
    outline: 0 none;
    border: 2px solid #289898;
    border-radius: 5px;
}

.form_auth input:hover+label,
.form_auth input:focus+label,
.form_auth input:valid+label {
    top: -9px;
    font-size: 12px;
}

.form_auth a.clear {
    display: none;
    position: absolute;
    box-sizing: border-box;
    width: 16px;
    height: 16px;
    margin: 14px 0 0 -27px;
    outline: none;
}

.form_auth input::-ms-clear {
    display: none;
}

/* :valid 表示输入合法时显示的样式 */

.form_auth input:valid ~ a.clear {
    display: inline;
}

/* + 是相邻兄弟选择器 */
/* ~ 相邻兄弟们选择器 */

/* 给有required字段的input添加样式 */
input:required {
    background: url() no-repeat right center;
}